Well, it depends upon who you talk to. Rheobus technically stands for rheostat bus, and a rheostat is basically a high-power pot (or variable resistor). However, some people use the term rheobus for a linear voltage regulator based fan speed controller (using an LM317, for instance).
But, the LM317 is no more efficient than a rheostat. The linear regulators dump just as much waste heat as a rheostat for a given voltage, they are simply cheaper. Also, the LM317 can maintain a steady voltage no matter how the load changes, but that really does no good for a fan bus; the load never changes. It looks more high tech, though.
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